This invention relates to nuclear magnetic resonance (NMR) apparatus.
Inserted (or implanted) NMR coils can be sited in many cases close to a lesion which it is proposed to treat by the application of an external therapy remotely targeted at the lesion. Many therapies of interventional magnetic resonance imaging (MRI) are thermal in nature, and involve energies which are intentionally destructive of tissue locally. Such therapies include r.f. hyperthermia, laser ablation and focussed ultrasound. These therapies depend on assumptions about tissue properties in assessing their exact energy profiles, and the location of the focus, or principal region of heat deposition. Because of uncertainties in in vivo temperature measurement, pilot energy depositions are not a satisfactory method of determining energy patterns in human subjects. It is possible to estimate spot location from external data but, short of heating up a region to see where it is, it is difficult to be sure it is where it was intended to be. Reflection, refraction and scattering can affect its location significantly. Heating hot enough to be seen reliably with MRI means that tissue is damaged, so that if there were an error in location, normal tissue might be unintentionally destroyed.
It has been proposed (EP-A-0 558 029) to provide a probe containing an MR coil with temperature and intensity sensors on the outside of a bulb which can be blown up around the probe to rest against the tissue outside. Then the maximum values of the temperature and intensity measured during ultrasonic wave treatment are compared with values calculated for the treatment, with a view to minimising the differences. However, the temperature and intensity sensors will measure the temperature and intensity adjacent them, but this is only indicative of the temperature and intensity at the intended focus of heat deposition, because temperature and intensity gradients are large and variable.